ON THE ROCKS Presents

Decapitated caldera and Devonian diamonds

July 25, 2026

Trip Leader: Dr. Judith Hannah, and Howard Coopersmith

Trip Coordinator: Tom Dimelow, David Schoderbek, Ron Paker

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Trip Description

Join us south of the Wyoming-Colorado state line (Livermore, CO) for a full day exploring two of the most geologically remarkable features in North America - the State Line Kimberlite District and the Virginia Dale Ring Dike Complex.

We'll kick off the morning with the Virginia Dale Ring-Dike Complex, joining CSU Professor Emerita Judith Hannah at this ancient, deeply eroded structure straddling the Colorado-Wyoming boundary. Intruded around 1.43 billion years ago. The complex is essentially the exposed root of a decapitated Precambrian caldera. The feature is roughly nine miles across and formed within the Sherman granitic batholith. It discordantly intrudes the surrounding metamorphic rocks. We'll explore its four main units: fringing Sherman granite, a composite zone of fragmented basaltic and granitic magmatically mixed rocks that appear in and around the ring dike complex and two ring-dikes of quartz monzogranite with interesting crystalline flow foliation fabric. We will drive through spectacular scenery and roadside outcrops that bring the whole structure to life along the way. We’ll also examine the central caldera containing a 1.39-billion-year Silver Plume irregular intrusion.

We'll spend the afternoon with kimberlite expert Howard Coopersmith at four separate Nix kimberlite diatremes. We will examine and possibly collect Ordovician - Silurian sedimentary rocks, upper and lower crustal Precambrian rocks and upper mantle nodules preserved within this Devonian-age serpentine matrix pipe. And who knows, even tiny diamonds!

Nix is one of roughly 35 kimberlite pipes and dikes in the State Line District. Kimberlites were first recognized in northern Colorado and adjacent Wyoming in 1964, with diamonds reported as early as 1975. Diamonds are known to occur in most of the district's diatremes, which range from circular and elliptical intrusions to dikes. These explosive eruptive features brought kimberlites to the Devonian surface from approximately 200 km in the mantle. Collectively, the district hosts over 100 kimberlitic intrusions that have yielded more than 130,000 diamonds since their discovery. A historic note - initial American commercial diamond mining took place from the Kelsey Lake kimberlite immediately south of the Wyoming border in 1994.

The Nix kimberlite - Virginia Dale ring-dike field trip participants plan to assemble at The Forks Restaurant in Livermore, CO prior to 9 AM. Orientation will commence at 9 AM with departure thereafter. The initial stops will be along US 287 approximately 11 miles north from Livermore at Virginia Dale, CO. There will be ample roadside parking for all stops. Bring a sack lunch and ample liquids, lunch break will take place at the Nix kimberlite stop. It is essential that participants carpool from the Denver area to Livermore. Additional carpool arrangements will take place there or at Virginia Dale. The trip will conclude no later than 4 PM.

Cost

• $55 for members
• $75 for non-members. (not a member? click here to join)
• Are you a student? email staff@rmag.org 

Registration 

Registration Closes July 20, 2026

Refunds for the July 25th, trip are only available until July 10, 2026

If you are unable to attend, your registration for all RMAG events are transferable. RMAG members may transfer their registration to another RMAG member, and non-members can transfer their registrations to whomever they wish. Should an RMAG member wish to transfer their registration to a non-member, the non-member would need to pay the balance between the member and non-member price.

Waivers

All registrants must fill out the Waiver and Release of Liability and Medical Emergency Form. Those who fail to complete these form will not be permitted to attend the field trip.